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4
Section
4
Water in the
Atmosphere
Water in the Atmosphere
Reading Focus
Objectives
After this lesson, students will be able to
I.2.4.1 Describe humidity and how it is
measured.
I.2.4.2 Explain how clouds form.
I.2.4.3 Name the three main types of clouds.
Key Concepts
• What is humidity and how is it
measured?
• How do clouds form?
• What are the three main types of
clouds?
Key Terms
Target Reading Skill
Asking Questions Explain that changing a
head into a question helps students
anticipate the ideas, facts, and events that
they are about to read.
Answers
Possible questions and answers include the
following:
How does the water cycle work? (Water
evaporates from the surface, condenses to form
clouds, and falls to Earth as rain or snow.)
What is relative humidity? (The percentage
of water vapor in the air compared to the
maximum amount air can hold at that
temperature.) How do clouds form? (Water
vapor in the air condenses to form liquid water
or crystals.) Can you tell about weather
conditions by looking at clouds? (Yes; each
type of cloud is associated with a different type
of weather.)
Teaching Resources
• Transparency I18
• water cycle • evaporation
• humidity • relative humidity
• psychrometer • condensation
• dew point • cirrus
• cumulus • stratus
Target Reading Skill
Asking Questions Before you
read, preview the red headings. In
a graphic organizer like the one
below, ask what or how questions
for each heading. As you read,
write answers to your questions.
The Water Cycle
Question
Answer
How does the
During the
water cycle work? water cycle . . .
How Does Fog Form?
1. Fill a narrow-necked plastic bottle with hot tap
water. Pour out most of the water, leaving
about 3 cm at the bottom. CAUTION: Avoid
spilling hot water. Do not use water that is so
hot that you cannot safely hold the bottle.
2. Place an ice cube on the mouth of the bottle.
What happens?
3. Repeat Steps 1 and 2 using cold water instead
of hot water. What happens?
Think It Over
Developing Hypotheses How can you explain your
observations? Why is there a difference between what happens
with the hot water and what happens with the cold water?
During a rainstorm, the air feels moist. On a clear, cloudless
day, the air may feel dry. As the sun heats the land and oceans,
the amount of water in the atmosphere changes. Water is
always moving between the atmosphere and Earth’s surface.
The movement of water between the atmosphere and
Earth’s surface is called the water cycle. As you can see in
Figure 13, water vapor enters the air by evaporation from the
oceans and other bodies of water. Evaporation is the process
by which water molecules in liquid water escape into the air as
water vapor. Water vapor is also added to the air by living
things. Water enters the roots of plants, rises to the leaves, and
is released as water vapor.
As part of the water cycle, some of the water vapor in the
atmosphere condenses to form clouds. Rain and snow fall from
the clouds toward the surface. The water then runs off the surface or moves through the ground, back into the lakes, streams,
and eventually the oceans.
Preteach
Build Background
Knowledge
L1
Water Vapor
Ask: When you take a shower, have you ever
noticed that the bathroom mirror clouds
up? (Yes.) Explain that when warm, moist air
from the shower comes into contact with the
cool surface of the mirror, the air cools and
can hold less water vapor. As a result, water
vapor condenses on the mirror. Point out
that clouds form in the same way: water
vapor on particles in the atmosphere
condenses when warm, moist air cools.
Skills Focus Developing hypotheses L1
Materials narrow-necked plastic bottle,
hot tap water, 2 ice cubes, cold tap water
Time 10 minutes
Tips Caution students to run hot water
slowly out of the taps. Make sure that
students let the bottle cool before
repeating Steps 1 and 2 with cold water.
Expected Outcome Fog will form in the
bottle when it contains hot water, but not
when it contains cold water.
Think It Over Fog forms when warm,
moist air rises from the surface of the hot
water and condenses as it cools near the ice
cube. This does not occur when the bottle
contains cold water because cold water
does not produce warm, moist air.
Humidity
How is the quantity of water vapor in the atmosphere measured? Humidity is a measure of the amount of water vapor in
the air. Air’s ability to hold water vapor depends on its temperature. Warm air can hold more water vapor than cool air.
Relative Humidity Weather reports usually refer to the
FIGURE 13
Water Cycle
In the water cycle, water moves
from oceans, lakes, rivers, and
plants into the atmosphere and
then falls back to Earth.
water vapor in the air as relative humidity. Relative humidity
is the percentage of water vapor that is actually in the air compared to the maximum amount of water vapor the air can hold
at a particular temperature. For example, at 10°C, 1 cubic
meter of air can hold at most 8 grams of water vapor. If there
actually were 8 grams of water vapor in the air, then the relative
humidity of the air would be 100 percent. Air with a relative
humidity of 100 percent is said to be saturated. If the air had
4 grams of water vapor, the relative humidity would be half, or
50 percent.
Condensation
Instruct
For: Water Cycle activity
Visit: PHSchool.com
Web Code: cfp-4024
Students can interact with the water cycle art
online.
For: Water Cycle activity
Visit: PHSchool.com
Web Code: cfp-4024
Precipitation
Evaporation
from plants
Evaporation
from oceans,
lakes, and streams
Surface runoff
Humidity
Teach Key Concepts
L1
Relative Humidity
Focus Show students a 5-gallon bucket. Tell
them that it can hold 5 gallons of water. Ask:
If the bucket held 2.5 gallons, what
percentage of the total amount that the
bucket can hold would this be? (50%) Tell
students that relative humidity is calculated
in a similar way.
Teach Ask: If one cubic meter of air at 10°C
can hold 8 grams of water vapor, what is the
relative humidity if it contains 2 grams of
water vapor? (25%) Tell students that
warmer air can hold more water vapor than
colder air can. Ask: How would the relative
humidity change if the air temperature
decreased? (It would go up.) Why? (The
amount of water in the air would be a greater
percentage of the amount that the colder air
can hold.)
Apply Ask: Why might condensation
occur if air cooled enough? (The amount of
water in the air would become greater than the
amount that the air could hold.)
Independent Practice
Teaching Resources
• Guided Reading and Study
Worksheet: Water in the Atmosphere
Student Edition on Audio CD
Differentiated Instruction
L1
English Learners/Beginning
Comprehension: Modified Cloze
Write some simple sentences on the board
that require the terms evaporation,
condensation, humidity, cumulus, stratus,
and cirrus. For example: “The amount of
moisture in the air is its _____.” Model
how to do the first item. As students read
through the section, have them fill in the
blanks. learning modality: verbal
English Learners/Intermediate L2
Comprehension: Modified Cloze Use
the same sentences described at left, but fill
in incorrect terms. Have students work in
pairs to determine the correct
answers. learning modality: verbal
Monitor Progress
L2
Drawing Ask students to draw a diagram of
the water cycle and label it with these terms:
evaporation, condensation, and
precipitation. Students can place their
drawings in their portfolios.
L2
Measuring to Find the Dew Point
Materials beaker, water at room
temperature, ice cubes, stirring rod,
thermometer
Time 10 minutes
FIGURE 14
Sling Psychrometer
A sling psychrometer is
used to measure relative
humidity.
Focus Review the definition of dew point.
Teach Have students fill a beaker with cool
tap water and record the temperature. Then
have them add ice cubes to the beaker and
stir. When moisture condenses on the
outside of the beaker, have students record
the temperature of the water again; this is the
dew point in the classroom.
Apply Ask: Would the dew point be the
same if you performed this experiment on
different days? (Probably not; the dew point
depends on the temperature and relative
humidity, which change from day to
day.) learning modality: logical/
mathematical
Math Skill Interpreting Graphs
Focus Have a volunteer explain how to
interpret the table. (The boldface numbers
represent the dry-bulb reading in degrees and
the difference in degrees between the two bulbs.
The other numbers represent the relative
humidity.)
Teach Tell students to subtract the wet-bulb
reading from the dry-bulb reading and find
the difference between the two readings in
the table.
Answers
1. 64%
2. 88%
3. It decreased from 18 degrees to
12 degrees.
4. It increased.
5. For the same amount of water in the air,
as the temperature decreases, the relative
humidity increases. Warm air can hold more
moisture than cool air can.
Measuring Relative Humidity Relative humidity
can be measured with an instrument called a
psychrometer. A psychrometer (sy KRAHM uh tur) has
two thermometers, a wet-bulb thermometer and a drybulb thermometer, as shown in Figure 14. The bulb of the
wet-bulb thermometer has a cloth covering that is
moistened with water. When the psychrometer is “slung”,
or spun by its handle, air blows over both thermometers.
Because the wet-bulb thermometer is cooled by
evaporation, its reading drops below that of the dry-bulb
thermometer.
If the relative humidity is high, the water on the
wet bulb evaporates slowly, and the wet-bulb temperature does not change much. If the relative humidity
is low, the water on the wet bulb evaporates rapidly,
and the wet-bulb temperature drops. The relative
humidity can be found by comparing the temperatures of the wet-bulb and dry-bulb thermometers.
What instrument measures relative
humidity?
Relative Humidity
Determining Relative Humidity
Relative humidity is affected by temperature.
Use the data table to answer the questions
below. First, find the dry-bulb temperature in
the left column of the table. Then find the
difference between the wet- and dry-bulb
temperatures across the top of the table. The
number in the table where these two readings
intersect indicates the relative humidity in
percent.
1. Interpreting Data At noon, the readings on a
sling psychrometer are 18°C for the dry-bulb
thermometer and 14°C for the wet-bulb
thermometer. What is the relative humidity?
2. Interpreting Data At 5 P.M., the psychrometer
is used again. The reading on the dry-bulb
thermometer is 12°C, and the reading on the
wet-bulb thermometer is 11°C. Determine
the new relative humidity.
3. Interpreting Data How did the temperature
change between noon and 5 P.M.?
Dry-Bulb
Reading
(°C)
Difference Between
Wet- and Dry-Bulb
Readings (°C)
1
2
3
4
5
10
88
76 65
54
43
12
88
78 67
57
48
14
89
79 69
60
50
16
90
80 71
62
54
18
91
81 72
64
56
20
91
82 74
66
58
22
92
83 75
68
60
4. Interpreting Data How did
relative humidity change during the course
of the day?
5. Drawing Conclusions How was the relative
humidity affected by air temperature?
Explain your answer.
3 Water vapor
condenses on tiny
particles in the air,
forming a cloud.
1 Warm, moist
air rises from the
surface. As air
rises, it cools.
2 At a certain
height, air cools to
the dew point and
condensation begins.
FIGURE 15
Cloud Formation
Clouds form when warm,
moist air rises and cools.
Water vapor condenses onto
tiny particles in the air.
How Clouds Form
When you look at a cloud, you are seeing millions of tiny water
droplets or ice crystals. Clouds form when water vapor in the
air condenses to form liquid water or ice crystals. Molecules
of water vapor in the air become liquid water in the process of
condensation. How does water in the atmosphere condense?
Two conditions are required for condensation: cooling of the
air and the presence of particles in the air.
What two factors are required for condensation to
occur?
mathematical
L2
Factors Causing Condensation
Focus Remind students that condensation is
one part of the water cycle.
Teach Explain that as warm, moist air rises
in the atmosphere, its temperature begins to
decrease. The rising air becomes saturated
and condenses, or reaches its dew point.
Ask: What happens at that point? (Clouds
form.) What else is needed for cloud
formation? (The water vapor must have a
surface on which to condense.)
Apply Ask students to explain why dew
forms on plants in the early morning.
(Dew forms when the ground reaches the
temperature at which water vapor
condenses.) learning modality: logical/
Teaching Resources
• Transparency I19
FIGURE 16
Condensation
Water vapor condensed on this
insect to form dew. Predicting
What would happen if the surface
were below freezing?
Differentiated Instruction
L3
Gifted and Talented
Graphing the Dew Point Have students
listen to local weather reports or check the
local newspaper to find the dew point and
relative humidity for several days. Ask
students to graph the relationships among
the dew point, temperature, and relative
humidity. learning modality: logical/
Teach Key Concepts
mathematical
The Role of Cooling As you have learned, cold air holds
less water vapor than warm air. As air cools, the amount of
water vapor it can hold decreases. The water vapor condenses
into tiny droplets of water or ice crystals.
The temperature at which condensation begins is called the
dew point. If the dew point is above freezing, the water vapor
forms water droplets. If the dew point is below freezing, the
water vapor may change directly into ice crystals.
The Role of Particles But something else besides a change
in temperature is needed for cloud formation. For water vapor to
condense, tiny particles must be present so the water has a surface
on which to condense. In cloud formation, most of these
particles are salt crystals, dust from soil, and smoke. Water vapor
also condenses onto solid surfaces, such as blades of grass or
window panes. Liquid water that condenses from the air onto a
cooler surface is called dew. Ice that has been deposited on a
surface that is below freezing is called frost.
How Clouds Form
Monitor Progress
L1
Less Proficient Readers
Understanding Concepts Select a
passage from the text, such as The Role of
Cooling or The Role of Particles. Read the
passage aloud as students follow along in
their books. After reading, ask some
questions about the content of the passage.
If students don’t know the answers,
challenge them to find them in the
passage. learning modality: verbal
L2
Writing Have students explain in their own
words how temperature, humidity, and dew
point are related.
Answers
Figure 16 Frost would form instead of dew.
A psychrometer
The cooling of air to the
dewpoint or below and the
presence of particles
Types of Clouds
Types of Clouds
Teach Key Concepts
L2
Classifying Clouds
Focus Refer students to Figure 17.
Teach Give students a few moments to
study the diagram, and then ask: What do
you notice about the relationship between
altitude and types of clouds? (Different types
of clouds form at different altitudes.) Explain
that cumulus clouds form during clear
weather when warm air rises over small
regions of Earth, such as parking lots,
because these areas are heated more by the
sun. Nimbostratus clouds are formed by
warm air rising over a wide area, so they tend
to cover the whole sky. Cumulonimbus
clouds form when a great deal of hot air rises
quickly and towers upward for several
kilometers. Strong winds at the bottom of
the stratosphere flatten the tops of
cumulonimbus clouds to give them their
characteristic anvil shape. Cirrus clouds are
formed high in the atmosphere, where it is
very cold and there is little water vapor,
making these clouds thin and wispy.
Apply Ask: What type of cloud might you
see when a thunderstorm is approaching?
(Cumulonimbus) What type of cloud are
you likely to see on clear, cold days? (Cirrus)
What type of weather can you expect when
you see dark, flat clouds covering most
of the sky? (Rain or snow) learning
modality: visual
Teaching Resources
• Transparency I20
Cirrus clouds
Clouds come in many different shapes, as shown in
Figure 17. Scientists classify clouds into three main types
based on their shape: cirrus, cumulus, and stratus.
Clouds are further classified by their altitude. Each type of
cloud is associated with a different type of weather.
Cirrus Clouds Wispy, feathery clouds are known as
Cumulus clouds
Stratus clouds
cirrus(SEER us) clouds. Cirrus comes from a word meaning a curl of hair. Cirrus clouds form only at high levels,
above about 6 kilometers, where temperatures are very low.
As a result, cirrus clouds are made of ice crystals.
Cirrus clouds that have feathery “hooked” ends are
sometimes called mare’s tails. Cirrocumulus clouds,
which look like rows of cotton balls, often indicate that a
storm is on its way. The rows of cirrocumulus clouds look
like the scales of a fish. For this reason, the term “mackerel
sky” is used to describe a sky full of cirrocumulus clouds.
Cumulus Clouds Clouds that look like fluffy, rounded
piles of cotton are called cumulus(KYOO myuh lus)
clouds. The word cumulus means “heap” or “mass” in
Latin. Cumulus clouds form less than 2 kilometers above
the ground, but they may grow in size and height until
they extend upward as much as 18 kilometers. Cumulus
clouds that are not very tall usually indicate fair weather.
These clouds, which are common on sunny days, are
called “fair weather cumulus.” Towering clouds with flat
tops, called cumulonimbus clouds, often produce thunderstorms. The suffix -nimbus means “rain.”
Stratus Clouds Clouds that form in flat layers are
called stratus(STRAT us) clouds. Recall that strato means
“spread out.” Stratus clouds usually cover all or most of
the sky and are a uniform dull, gray color. As stratus
clouds thicken, they may produce drizzle, rain, or snow.
They are then called nimbostratus clouds.
What are stratus clouds?
FIGURE 17
(km)
13
Clouds
Cirrus
The three main types of clouds
are cirrus, cumulus, and stratus.
A cloud’s name contains clues
about its height and structure.
Interpreting Diagrams What
type of cloud is found at the
highest altitudes?
12
11
10
Cirrocumulus
9
8
Altocumulus
7
Cumulonimbus
6
Altostratus
5
4
Observing How Clouds Form
L2
Materials water, clean gallon bottle with cap,
bicycle pump, nail
Time 10 minutes
Focus Review how clouds form by
condensation.
Teach Explain that air gets warmer when it
is compressed and cooler when it is allowed
to expand. Cover the bottom of a gallon
bottle with a few centimeters of water. Use a
nail to punch holes in the cap until you have
an opening about 0.5 cm in diameter. Place
the cap on the bottle, and push the nozzle of
a bicycle pump into the opening. Ask a
volunteer to push down on the pump two or
three times. Quickly release the cap, and a
cloud will form inside the bottle.
Apply Ask: Why did the cloud form inside
the bottle? (Pumping air into the bottle
compressed and warmed the air in the bottle,
so it picked up moisture from the water.
Releasing air from the bottle allowed the air in
the bottle to expand and cool so that it could
hold less water. Water condensed out of the air
that remained in the bottle, forming a
cloud.) learning modality: visual
3
Cumulus
2
Nimbostratus
Stratus
1
Fog
Differentiated Instruction
L1
Special Needs
Recognizing Cloud Shapes Help
students become more familiar with the
distinctive shapes of the main cloud types.
Display several drawings or pictures of the
cloud types. Point out their distinctive
shapes, and challenge students to identify
them. learning modality: visual
L1
Less Proficient Readers
Identifying Clouds Explain that the
words cumulus, stratus, and cirrus describe
basic cloud shapes. The word nimbus or
the prefix nimbo- means that the cloud
produces rain or snow. The prefix altomeans that the cloud is a medium-altitude
cloud. The prefix cirro- means that the
cloud is a high-altitude cloud. learning
modality: verbal
Monitor Progress
L2
Drawing Have students draw and label each
of the three main cloud types. Have students
save their drawings in their portfolios.
Answers
Figure 17 cirrus
Uniformly dull, gray clouds
that form in flat layers
Monitor Progress
Altocumulus and Altostratus Part of a cloud’s
name may be based on its height. The names of
clouds that form between 2 and 6 kilometers above
Earth’s surface have the prefix alto-, which means
“high.” The two main types of these clouds are
altocumulus and altostratus. These are “middle-level”
clouds that are higher than regular cumulus and
stratus clouds, but lower than cirrus and other
“high” clouds.
L2
Answers
Figure 18 The heat of the sun will cause the
water droplets in fog to evaporate.
Clouds that form at or near
the ground
Assess
Reviewing Key Concepts
1. a. A measure of the amount of water
vapor in the air b. Humidity is the actual
amount of water vapor in the air. Relative
humidity is the percentage of water vapor
actually in the air compared to the total
amount of water vapor that the air can hold.
c. 20%
2. a. Condensation b. Air must be cooled
to its dew point, and particles must be
present in the air. c. When the dew point is
below freezing, ice crystals might form.
3. a. Cumulus, stratus, and cirrus
b. Possible response: Cumulus clouds look
like fluffy, rounded piles of cotton. Stratus
clouds form in flat layers. Cirrus clouds are
high, wispy, and feathery. c. Low-level
clouds are fog, cumulus, stratus, and
nimbostratus. Medium-level clouds are
altocumulus and altostratus. High-level
clouds are cirrostratus and cirrus.
Reteach
L1
Have students look at the diagram of
the water cycle and explain each of the
steps in detail.
Performance Assessment
L2
Have students infer why they can see their
breath on a cold day. (Water vapor from their
warm, moist breath condenses when it enters
the cold air.)
FIGURE 18
Fog Around the Golden Gate Bridge
The cold ocean water of San Francisco Bay is
often covered by fog in the early morning.
Predicting What will happen as the sun rises
and warms the air?
Fog Clouds that form at or near the ground are
called fog. Fog often forms when the ground cools
at night after a warm, humid day. The ground cools
the air just above the ground to the air’s dew point.
The next day the heat of the morning sun “burns”
the fog off as its water droplets evaporate. Fog is
more common in areas near bodies of water or lowlying marshy areas. In mountainous areas, fog can
form as warm, moist air moves up the mountain
slopes and cools.
What is fog?
4
Section 4 Assessment
Target Reading Skill
Asking Questions Use the answers to the
questions you wrote about the headings to help
answer the questions below.
Reviewing Key Concepts
1. a. Reviewing What is humidity?
b. Comparing and Contrasting How are
humidity and relative humidity different?
c. Calculating Suppose a sample of air can at
most hold 10 grams of water vapor. If the
sample actually has 2 grams of water vapor,
what is its relative humidity?
2. a. Identifying What process is involved in
cloud formation?
b. Summarizing What two conditions are
needed for clouds to form?
c. Inferring When are clouds formed by ice
crystals instead of drops of liquid water?
3. a. Listing What are the three main types of
clouds?
b. Describing Briefly describe each of the
three main types of clouds.
c. Classifying Classify each of the following
cloud types as low-level, medium-level, or
high-level: altocumulus, altostratus,
cirrostratus, cirrus, cumulus, fog,
nimbostratus, and stratus.
Water in the Air Fill a large glass half full with
cold water. Show your family members what
happens as you add ice cubes to the water.
Explain to your family that the water that
appears on the outside of the glass comes
from water vapor in the atmosphere. Also
explain why the water on the outside of the
glass only appears after you add ice to the
water in the glass.
Teaching Resources
• Section Summary: Water in the
Atmosphere
• Review and Reinforce: Water in the
Atmosphere
• Enrich: Water in the Atmosphere
Water in the Air L1 Tell students to
use cold tap water, not cold water from the
refrigerator, which could cause the water
to condense on the outside of the glass
without the addition of ice. The water on
the outside of the glass condenses from
water vapor in the air. It appears only after
ice is added because water vapor begins to
condense out of the air when the
temperature reaches the dew point.